1. Field of Invention
The present invention relates to a magnetometer. it is used in the precise measurement of weak magnetic fields (typically in the range 20 to 70 .mu.T corresponding to the values of the earth's magnetic field).
2. Discussion of Background
The magnetometer according to the invention concerns a so-called resonance magnetometer type, a description of such a type is given in the article by F. HARTMAN entitled "Resonance Magnetometers", published in the journal "IEEE Transactions on Magnetics", vol. MAG-8, No. 1, March 1972, pp. 66-75.
A resonance magnetometer is an apparatus which, immersed in a magnetic field Bo, supplies an electric signal of frequency F, whose value is linked with Bo by the so-called LARMOR relation: EQU F=.gamma.Bo
where .gamma. is a gyromagnetic ratio (of an electron or nucleon as a function of the substance used). For example, for the electron this ratio is equal to 28 Hz/nT.
The excitation of the magnetic resonance is obtained by a winding positioned around the substance used and the sampling of a resonance signal takes place either by another winding (electrical variant) or by a pumping light beam (optical variant).
Although the invention is applicable to all magnetic resonance magnetometers, the optical pumping magnetometer will more particularly be considered hereinafter, but without in any way restricting the scope of the invention.
An optical pumping magnetometer is diagrammatically shown in FIG. 1. An at least partly transparent cell 10 is filled with a gas 12, which is generally helium, at a pressure of 1 to a few Torr. A light source 14 supplies a light beam 18, whose wavelength is approximately 1.1 .mu.m. This beam is appropriately polarized by a means 16 and is then injected into the cell 10.
In addition, a radio frequency or high frequency discharge of a so-called weak or gentle type is produced in the gas by a generator 30 connected to two electrodes 32,33 arranged around the cell 10. This discaharge produces atoms in a metastable state (2.sup.3 S.sub.1 in the case of helium). The incident light beam "pumps" these atoms from the metastable state to bring them into another excited state (2.sup.3 P).
In the presence of a magnetic field Bo, the energy levels are subdivided into so-called ZEEMAN sublevels. A resonance between such sublevels can be established by a high frequency field (magnetic resonance) or by light modulation (double optical resonance; COHEN, TANNOUDJI, Ann. Phys. 7, 1962, p. 423). In the case of isotopic helium 4, the resonance is established between two ZEEMAN electronic sublevels of the metastable state.
This resonance is revealed by various known means, whereof one variant is shown in FIG. 1. It is a winding 20 located on either side of the cell 10 (in a so-called HELMHOLTZ arrangement) of a high frequency generator 22, a photodetector 24 receiving the light radiation which has traversed the cell, an amplifier 25, a synchronous detection means 21 and an integrator 23. The means 21 to 26 will be designated CC hereinafter. The generator 22 supplies the winding 20 with current at frequency F, which creates an oscillating magnetic field, whereof one component maintains the resonance and on its return modulates the light frequency which is passed through the cell, said modulation constituting the signal. It is revealed by the synchronous detection at the output of the photodetector, via the amplifier. The reference is given by the generator. The synchronous detection output corresponding to the component of the signal is in phase with the reference serves as an error signal. The static error is eliminated therefrom by the integrator. This error signal readjusts the frequency F of the synthesizer to the LARMOR frequency. For this purpose it is necessary for the synthesizer to be voltage-controllable and it can also be replaced by a voltage-controlled oscillator (V.C.O).
An electric resonance signal S is consequently established in this loop at the LARMOR frequency. A frequency meter 26 gives the value F thereof. The field to be measured Bo is deduced therefrom by the relation Bo=F/.gamma..
Helium magnetometers of this type have firstly made use of helium lamps. The recent arrival of lanthanum-neodymium aluminate (or LNA) crystals have made it possible to produce lasers tunable about the wavelength of 1.083 .mu.m precisely corresponding to the optical pumpling line of helium. Naturally this type of laser has taken the place of these lamps with a significant performance improvement, so that once again interest have been aroused in such magnetometers. Such a magnetometer equipped with a LNA laser is described in FR-A-2 598 518.
Although satisfactory in certain respects, these magnetometers still suffer from disadvantages. The most important disadvantage is the existance of a frequency shift due to the so-called BLOCH-SIEGERT effect caused by one of the components of the high frequency exciting field. This phenomenon is illustrated in FIG. 2, which shows the respective orientations of the magnetic fields used.
Ideally, the radio frequency or high frequency field B1 should be perpendicular to the field Bo to be measured. In practise, these two fields form an angle between them of .theta.. The component of B1 projected onto a plane perpendicular to Bo consequently has for the amplitude B1 sin .theta.. The amplitude of the resonance signal is consequently dependant on the angle between the field to be measured and the high frequency field, the magnetometer not being isotropic in amplitude.
The alternating field B1 sin .theta. can be considered as the result of the composition of two circular components b.sup.+ 1 and b.sup.- 1 rotating in opposite directions. Only one of these components (b.sup.+ 1 in FIG. 2) rotates in the precession direction of the spins Sp and is able to maintain the precession about the field Bo. The other component b.sup.- 1 does not directly participate in the resonance phenomenon, but induces a frequency shift of value: ##EQU1## This is the BLOCH-SIEGERT effect described in the journal "Physical Review", 57, 1940, p.522.
When the orientation of the cell (with its winding) varies, the angle .theta. varies and the frequency shift changes and the magnetometer is not isotropic in frequency.
In order to illustrate the extent of this phenomenon, it is possible to consider the case of a helium 4 magnetometer (for which the resonance is electronic, the nucleus of such isotope being spin-free.) located in a diameter 6 cm sphere. the high frequency field B1 is created by two 4.5 cm diameter coils in the HELMHOLTZ position and 2.25 cm apart. They are constituted by three 0.5 mm diameter, copper wire turns. When they are traversed by a 10 mA current, the field at the centre of the cell is 600 nT. In our latitudes, the earth's magnetic field Bo is 70 .mu.T.
If .theta.=0.degree., the high frequency field B1 is parallel to the field Bo and there is no resonance signal. If .theta.=90.degree., the high frequency field B1 is perpendicular to the field Bo, the resonance signal is at a maximum, but so is the BLOCH-SIEGERT effect. The frequency shift is 55 Hz, i.e. an error on the field Bo to be measured of 2 nT (approx. 3.10.sup.- 5). If .theta.=45.degree., the resonance signal is only 70% of its maximum value and the frequency shift is 40 Hz, i.e. an error on the field to be measured of 1.4 nT.
This frequency anisotropy can be corrected by compensation formulas, if it is possible to determine the direction of the static field Bo relatively to the sensor using appropriate means (e.g. a directional magnetometer). However, this solution is costly and difficult to put into effect, because all instrumentation must be amagnetic and rigidly linked with the cell.